VLBI Observations of OH stars: S Persei

Abstract The masers in three stars, S Per, OH53.6-0.2 and VXSgr, were observed in the main lines of OH at λ 18 cm in March 1997 with a VLBI array consisting of 6 EVN antennas and 3 in the USA. Both S Per and VX Sgr were detected on transatlantic baselines, showing that at least some of the maser spots are very compact with sizes less than around 2 mas. The observations confirm that in S Per there is main line OH emission close to star at a similar distance (≈ 100 AU) as the H 2 O shell. The most blue-shifted emission comes from the centre of the object suggesting radial amplification of the maser radiation, whilst most of the remainder comes from a thick shell, some 100 AU from the star. This contrasts with the 1612 MHz OH emission that comes from an outer shell some 1000 AU from the star. Estimates of the magnetic fields in the shell from Zeeman pairs in S Per show fields of a few hundred nT.